Conventional offshore production facilities often have a floating facility or fixed platform stationed at the water's surface and subsea equipment, such as a Christmas tree or other type of wellhead, positioned over the subsea wells at the mud line of a seabed. The production wells drilled in a subsurface formation typically produce fluids, such as oil, gas, and water, to the subsea wellhead. Produced fluids from multiple wells may be comingled through a manifold. This fluid (wellbore fluid) may be carried to a surface vessel, such as a Floating Production Storage and Offloading vessel (FPSO) or other vessel, to a fixed platform, or to shore for processing. However, processing in the vicinity of the well head provides a number of advantages. For example, separation of natural gas and water from the hydrocarbon stream before cooling may reduce the formation of hydrates in the production lines. In other examples, subsea units may be used to inject additives and other materials into production lines or wellbores.
Physical parameters of the subsea equipment can be monitored to adjust the operation. In one example, the measurement of pressure and temperature in a well's annulus may be used to monitor annulus pressure integrity. In a second example, the levels in a subsea separation unit may be used to adjust flow rates of fluids entering and exiting the equipment. Currently, parameter data may be monitored by sensors connected directly to a subsea control module (SCM) by electrical leads. The data is then sent via communications lines in umbilical lines that reach from the surface or from onshore centers. The number of lines available for monitoring is pre-determined and thus fixed. Accordingly, the flexibility of the systems may be limited. Further, the leads and connections may be vulnerable to water infiltration.
Fiber optic lines have been used for measuring parameters of systems using the detection of acoustic signals by changes in an optical signal in the fiber optic line. For example, International Patent Publication No. WO 2013/045941 discloses a system for monitoring the flow of fluids within conduits, such as pipes, using a fiber optic system based on distributed acoustic sensing (DAS). A disclosed method can determine flow rate or flow regime within a conduit, such as within a production well or a pipeline. Embodiments involve introducing an acoustic stimulus into the fluid within the conduit. An optical fiber deployed along the path of the conduit is interrogated to provide a DAS sensor. The acoustic signal detected by a plurality of channels of the DAS sensor is analyzed to determine at least one flow characteristic. Analyzing the acoustic signal includes identifying reflections of the acoustic stimulus caused by the fluid within the conduit and analyzing the reflections to determine any Doppler shift.
Other techniques have been proposed for using acoustic coupling with fiber optics to communicate with sensors. For example, U.S. Patent Application Publication No. 2012/0017687 discloses a remote sensing system. In the remote sensing system, a plurality of sensors output information into a DAS system through acousto-mechanical signals. The sensors are indirectly coupled by the acousto-mechanical signal to an optic fiber at the center of the DAS system, wherein the acousto-mechanical signal is transmitted through an intermediary body, such as the ground or a conduit.
More effective and flexible communications between subsea units and control sites is desired as these types of units become more common and more complex.